PROJECT SUMMARY/ABSTRACT Aging-associated brain disorders, including cognitive decline, are among the greatest public health challenges. But without understanding the basis of age-related brain disorders at the molecular, circuit, and systems levels, effective therapeutic strategies cannot be developed. DNA repair is emerging as a potential regulator of age- related cognitive decline and neurodegeneration. The brain may be vulnerable to genomic alterations due to its network structure, the complexity of its transcriptome, and the low or absent turnover and long lifespan of neural cell types. This suggests genome maintenance pathways are crucial for brain health: persistent or incorrectly repaired DNA double-strand breaks (DSBs) could contribute to genomic alterations, thus promoting age-related cognitive impairment and neurodegenerative disorders. The role, however, of post-developmental neuronal and astrocytic DSB repair in brain physiology and maintenance of brain function with age has not been addressed. Moreover, theories of cognitive decline have focused on potential age-related changes in neuronal function, neglecting consideration of astrocytes and the complete neuro-glio-vascular circuit. The broader implication for these fundamental gaps in knowledge is that crucial opportunities for development of therapeutics for treatment and prevention of brain disorders may be missed. This provides a strong rationale for elucidating the biology of neuronal and astrocytic DSB repair at multiple levels. Thus, our long-term goal is to determine the extent to which neural DNA double-strand break formation and repair impact brain function and disorders. This application proposes to elucidate the relationship between systems-level neural circuit function and the DNA repair machinery in neurons and astrocytes with age. The central hypothesis of the proposed project is that DNA double-strand break formation and repair in mature neurons and astrocytes impact neural physiology. To test this hypothesis and to advance toward our long-term goal, we propose the following specific aims: (1) Determine impact of classical non-homologous end-joining DNA repair on neuronal physiology; (2) Elucidate role of astrocytic DSB repair in circuit homeostasis and maintenance of neural function during aging; and, (3) Elucidate non-canonical, homology-directed DSB repair pathways in neurons. The proposed approach involves a comprehensive, multidisciplinary analysis of neuronal and astrocytic function at the genetic, organismal, and circuit level. The proposed project is significant because it will use innovative approaches to investigate emerging concepts with major implications for human brain health, age-related cognitive decline, and neurodegenerative diseases. The project is further significant because it will refine and develop new research tools and models. Insights gained from the proposed studies are also expected to inform research and knowledge in other fields related to genomic stability and aging.